An increasing word population leads to a continually increasing amount of refuse. Additionally, an increased level of civilization appears to generate an increased amount of refuse on a per capita basis. Both factors in combination lead to mounting pressure to devise methods of waste disposal which are economically, energetically, and environmentally sound.
In recent years, especially in urban areas, the increased demand for usable land and other concerns has caused one to turn from a landfill as the major mode of refuse disposal to other options, especially the use of raw refuse as an energy source. One variant of the latter is the mass burning approach, where all the refuse in its raw state is burned without any preliminary treatment such as separating the noncombustible from combustible material. Quite briefly, in this method raw garbage is dumped into storage where it is homogenized and dried to some degree. Refuse from the storage area is fed into a combustion zone where the heated gases often are used to generate steam. Flue gases then pass from the combustion zone to a separation zone, often an electrostatic precipitator, where dust and ash are removed. The ash so removed from the flue gas, called fly ash is then mixed with the ash collected in the combustion zone, and the combined ash used for landfill, in road construction, and so forth.
It is well known that some of the more volatile compounds of certain metals tend to accumulate in the fly ash. Especially where the latter is to be used as landfill, leaching of toxic metals, especially cadmium and lead, constitutes a potential hazard to the ecosystem, for example, both surface water supplies and aquifers. The Environmental Protection Agency (EPA) has promulgated a procedure to determine the toxicity of solid wastes, and where residues exceed the toxicity as stated in the Federal Register Code 40, No. 26124, the waste is classified as a hazardous waste requiring control under the Hazardous Waste Management System. A recent report prepared for the Office of Solid Waste, U.S. Environmental Protection Agency, which was a limited survey of several kinds of solid waste, seems to suggest that levels of cadmium and lead in fly ash pose perhaps the most serious environmental threat, and that such fly ash alone would need to be treated as a hazardous waste. EP Toxicity Test Results on Residues from Eight Resource Recovery Facilities, SYSTECH Corporation, February, 1981.
The environmental hazard of fly ash containing amounts of cadmium and lead greater than the toxic levels specified by the EPA is somewhat diminished by mixing such ash with heavy ash, such that the resulting landfill mixture is within the toxic levels for the cited metals. Nonetheless, it is highly desirable to reduce the amount of cadmium and lead reched from fly ash and other solid waste to an amount below the toxic levels specified by the EPA. The invention herein is a solution to this problem. More specifically it is a method of treating dry, solid residues, especially fly ash, so as to reduce the amounts of cadmium and lead leached from such residues to a level below the toxic level specified by the EPA. Stated differently, the invention herein is a method of immobilizing, or insolubilizing, cadmium and lead in solid waste. The method is convenient, quite simple, very efficient, and relatively low cost. The method is, therefore, commercially extraordinarily attractive as well as being environmentally beneficial.
The precipitation of heavy metals, including cadmium and lead, as their sulfides is a well-known analytical technique. Removal of soluble cadmium and lead from waste water by precipitation of cadmium sulfide and lead sulfide following addition of sulfide ion has been described as an effective treatment. Bhattacharyya et al., Separation Science and Technology, 14, 441-52 (1979). Solid wastes containing cadmium and lead were treated with 3-15% calcium hydroxide and/or magnesium sulfate, the pH was adjusted to 8-10.5, and the solid coated with asphalt to prevent the leaching of cadmium and lead. Chemical Abstracts, 92; 185414d. The preceding method appears to be a mixture of coagulation-flocculation followed by encapsulation in a hydrophobic, petroleum-based solid.
The invention herein is based on the discovery that insolubilization of lead and cadmium in fly ash or similar dry solid residues is effectively brought about by the addition of small amounts of aqueous sulfide. A further discovery is that calcium sulfide is especially effective for the immobilization of lead and cadmium. A still further discovery is that treatment of fly ash and similar dry solid residues with dry lime followed by a solution of an alkali metal sulfide in an amount barely sufficient to moisten the solid refuse effectively simulates the action of calcium sulfide in immobilizing lead and cadmium.